Low-pressure mercury vapor discharge lamp



Oct. 1967 D. KOLKMAN ET AL 3,349,274

' LOW-PRESSURE MERCURY VAPOR DISCHARGE LAMP Filed Feb. 25, 1965 45 -s Q5+12% T ADOLF .1. DE VISSER BY ik/L fi' United States Patent Ofiice p3,349,274 Patented Oct. 24, 1967 Claims priority, applicationNetherlands, Mar. 6, 19s4,

e r-2,292 8 Claims. (Cl. 313-109) ABSTRACT OF THE DISCLOSURE Alow-pressure mercury vapor discharge lamp employing a starting stripbetween the envelope and the layer of luminescent material. A portion ofthe layer of luminescent material extending between an electrode and apoint between the electrodes which point is not more than one-fifth thedistance between the electrodes is either removed to expose the startingstrip or a con- .ductive starting strip is provided over the layer inorder to facilitate ignition at both high and low temperatures.

The invention relates to low-pressure mercury vapor discharge lampshaving electrodes arranged at the ends of an elongate discharge space, aluminescent layer and a conducting strip extending between theelectrodes.

One of the problems involved in the use of low-pressure mercury vapordischarge lamps is that for the ignition a voltage is required which ishigher than the normal operating voltage, that is to say, the voltagemeasured between the electrodes of the normally operating lamp. Thisproblem arises in particular when the length of the discharge space isso great that the ignition voltage is higher than the mains voltage forwhich the lamp is intended. Many methods have been tried to obviate thisdifiiculty. One of these methods consists in that a so-called ignitionstrip, that is to say, a comparatively narrow conducting strip, isprovided in the lamp and extends between the electrodes. In thedischarge lamps having a luminescent layer applied to a support, forexample, the wall of the lamp, to which lamps the invention relates, theignition strip may be provided on the luminescent layer and mayconsequently be exposed to the discharge, or it may be provided betweenthe luminescent layer and the support thereof.

It is :known to connect such ignition strips to an electrode, Thismethod has a grave disadvantage in that it is difficult to make theconnection in practice, since in the manufacture of low-pressure mercuryvapor discharge lamps, first the luminescent layer and the ignitionstrip are provided; subsequently, the electrodes, which are mounted on aso-called stem mount, are introduced into, I

the lamp, whereupon the mount is sealed to the envelope of the lamp.Consequently, a connection must be made between the electrodes and theignition strip by means of long flexible 'wires or by means of resilientmembers. A discharge lamp having a non-connected strip does not sufferfrom this limitation and is therefore much more attractive; itsmanufacture is only slightly more complicated than that of a lampwithout an ignition strip.

Investigations which have led to the invention have proved that there isa great difference between the behaviour of discharge lamps which havean internal nonconnected ignition stripwhich is disposed between theluminescent layer and the support, and that of lamps which have anon-connected internal ignition strip which is provided on theluminescent layer and is thus in electrically conducting contact withthe discharge plasma. This is apparent inter alia from the form and thelocation of the characteristic curves which indicate the relationshipbetween the ignition voltage and the temperature of the lamp after theelectrodes have reached the emission temperature. One of the factorswhich determine this lamp temperature is the ambient temperature. It isknown that the ignition voltage is a minimum at a given temperature andincreases both at lower and at higher temperatures. Tests with ignitionstrips of equal dimensions have shown that with the use of stripsdisposed between the luminescent layer and the support thecharacteristic curves have approximately the same form as thecharacteristic curves obtained with the use of strips provided on theluminescent layer, but as a whole they lie at lower temperatures andoften show a slightly higher minimum value of the ignition voltage. Thismeans that lamps in which the ignition strip is provided on theluminescent layer ignite less readily at low temperatures but lamps inwhich the ignition strip is disposed between the luminescent layer andthe support ignite less readily at high temperatures.

It is advantageous to construct the lamps so that they have a lowignition voltage over a wide temperature range, which guarantees theirignition and re-ignition at widely different ambient temperatures.

To this end, lamps have already been manufactured in which the wholeinner side of the envelope is provided with a conductive coating.However, this necessitates the use of transparent conducting layers. Inmost cases tin ture range. This method has a serious disadvantage,

however; dark points and spots are produced on the wall of the tube andon the layers applied thereto already after a few hours operation. Inorder to mitigate this disadvantage, it has been suggested to removepart of the luminescent layer, for example, at the ends located near theelectrodes. It has been found, however, that the improvement obtained inthis manner is negligible.

In investigations which ultimately have led to the invention an ignitionstrip was provided both on and under the luminescent layer. If these twostrips were provided one above the other, the lamp behaved like a lamphaving an ignition strip provided on the luminescent layer; if theignition strips were not disposed exactly above each other, the ignitionvoltage found at lower temperatures was always higher than that of alamp having an ignition strip provided under the luminescent layer,while at higher temperatures this voltage was higher than that of a lamphaving an ignition strip provided on the luminescent layer.

A low-pressure mercury vapor discharge lamp in accordance with theinvention having electrodes arranged at the'ends of an elongatedischarge space, a luminescent layer and a conducting strip from 1 to 10mm. wide, which strip extends between the electrodes but is notconnected thereto and is disposed between the luminescent layer and itssupport, is characterized in that part of the strip between at least oneof the electrodes and a point spaced from this electrode by a distancewhich is smaller than one fifth of the distance between the electrodesis in electrically conducting contact with the discharge plasma.

There are various ways of establishing the electrically conductingcontact between the ignition strip and the discharge plasma. Forexample, part of the ignition strip According to a particular embodimentof the invention, the conducting contact between the ignition strip andthe discharge plasma may also be established in that a conductingmaterial, in particular a conducting material identical with that of theignition strip, is applied to the above described desired part of theluminescent layer which is located above the ignition strip. For it hasbeen found that in this case during the thermal treatment to which thelamps are always subjected after the application of the luminescentlayer, a sufliciently conducting contact is established between theignition strip disposed under the luminescent layer and the conductingmaterial applied along a short distance to the luminescent layer.

Since in a lamp in accordance with the invention the width of theconducting strip is limited, it is possible to use both transparent andopaque ignition strips. Although with wider ignition strips of otherwiseequal structure the ignition voltage obtained is always lower, the useof the invention with strips having a width exceeding mm. is notexpedient, since the reduction of the ignition voltage with respect tothat of a lamp having a strip of 10 mm. width located under theluminescent layer is too small. Moreover, with transparent strips havinga width exceeding 10 mm. the above-described formation of spots becomesinconvenient, while with the opaque strips the loss of light isexcessive. When opaque material is used, the width of the strippreferably does not exceed 4 mm.

It is not necessary for both ends of the ignition strip located near theelectrodes to be in contact with the discharge plasma. In some cases anelectrically conducting contact at one end may be sufficient.

The length of the part of the ignition strip which is in electricallyconducting contact with the discharge plasma is limited to one fifth ofthe distance between the electrodes. If the length of this part shouldbe greater, one comes near to those constructions in which the wholestrip is exposed to the discharge or in which ignition strips areprovided above each other both on and under the luminescent layer. Asstated hereinbefore, both constructions are unfavorable.

The length of the part of the ignition strip which is in electricallyconducting contact with the discharge plasma preferably is between 4 mm.and mm.

The shortest distance between an electrode and the part of the ignitionstrip which is in electrically conducting contact with the dischargeplasma preferably is less than 20 mm. If this distance is made greater,the form of the ignition characteristic curve is approximatelymaintained, it is true, but the ignition voltage increases at the samelamp temperature.

It has been found that in lamps in accordance with the invention it isadvantageous when the ignition strip does not extend beyond the partwhich is in electrically conducting contact with the discharge plasma.The ignition voltage of these lamps is then mostly slightly lower (byabout 5 v.) than with an ignition strip which extends beyond said part.

The invention will now be described more fully with reference to theaccompanying drawing, in which:

FIG. 1 and FIG. 1a are respectively a longitudinal sectional andcross-sectional view of a discharge lamp in accordance with theinvention;

FIG. 2 shows a particular embodiment of a lamp in accordance with theinvention;

FIG. 3 shows a graph of the relationship between the ignition voltageand the lamp temperature for lamps in accordance with the inventionwhich have different ignition strips.

In FIG. 1, reference numeral 1 denotes the envelope, made for example ofglass, which encloses the elongate discharge space 2 of a low-pressuremercury vapor discharge lamp; in this space are disposed electrodes 3and 4. Reference numeral 5 designates the luminescent layer which alongthe greater part of the circumference bears directly on the envelope.However, an ignition strip 6,

which is not connected with the electrodes 3 or 4, is locally interposedbetween the layer 5 and the wall 1. In the proximity of the electrode 3a window 7, which extends from a point 8 to a point 9, is provided inthe luminescent layer 5 so that at this area the ignition strip 6 is inelectrically conducting contact with the discharge plasma in the space2. The distance between the point 9 and the electrode 3 always is lessthan one fifth of the distance between the electrodes 3 and 4. Theshortest distance between the electrode 3 and the window 7 is preferablyless than 20 min. Consequently, the length from point 8 to point 9 andthe location of the window are restricted within limits which aredetermined by the structure of the lamp and more particularly by thelocation of the electrodes. The distance between point 8 and point 9preferably is between 4 mm. and 40 mm. With a shorter distance, thedecrease of the ignition voltage generally is not sufficient at highertemperatures; for reasons of manufacture, a distance greater than 40 mm.is not particularly attractive and usually not necessary for attainingthe object of the invention. The width of the window 7, measured alongthe circumference, may be greater than the width of the conducting strip6. As is clearly apparent from the drawing, the ignition stripterminates at the point 8.

FIG. 2 shows a lamp which for the greater part corresponds to that ofFIG. 1. Instead of the window of FIG. 1, a small surface 11 ofconducting material is provided on the luminescent layer opposite partof the conducting strip 10. This material is preferably identical withthat of the ignition strip 10. What was said with reference to FIG. 1inter alia about the length, the width and the place of the window 7,with due alterations applies to the conducting surface 11 also.

The material of the ignition strip may be constituted by a great manysubstances or mixtures of substances which have already been suggestedfor this purpose. The ignition strip may also be provided in a knownmanner. A particularly suitable mixture of substances is obtained bymixing 680 gms. of graphite powder, 4500 mls. of butylacetate, 300 mls.of monoethylglycolether, gms. of nitrocellulose and 2040 gms. ofpowdered glass having a grain size of 0.075 mm. Part of theabove-described suspension is applied to the wall in the form of a stripby means of a tool suitable for this purpose.

In the graph of FIG. 3, the ignition voltage of a lowpressure mercuryvapor discharge lamp is plotted along the ordinate, while the lamptemperature in degrees centigrade is plotted along the abscissa. Thecharacteristic curves shown in the graph were measured on a 40 w. lampin which the distance between the electrodes was approximately cms. andthe strip had a total resistance of approximately 4000 0.

Curve 1 gives the relationship between the ignition voltage and the lamptemperature with the use of an ignition strip 2 mm. wide provided on theluminescent layer (consequently not in accordance with the invention).As is apparent from the curve, the ignition voltage is lowest atapproximately 45 C. and increases both at higher and at lowertemperatures. Especially at lower temperatures, the ignition voltagerises steeply. Over the range between 205 v. and 220 v., which is themost important range in practice, ignition is obtained at ambienttemperatures of from 25 to 15 C.

Curve 2 applies to a strip which is 2 mm. wide and is disposed entirelyunder the luminescent layer (consequently not in accordance with theinvention). As is apparent from the figure, the ignition voltage in thiscase is considerably higher at higher temperatures than that of the lampin which curve 1 was measured. At low temperatures such a lamp operatessatisfactorily, but this is no longer the case at temperatures higherthan approximately 20 C.

Curve 3 was measured on a lamp in which a 2 mm. w de ignit on strip isprovided on the luminescent layer and a 2 mm. wide ignition strip isdisposed under this layer. The distance between the strips wasapproximately 90 measured along the circumference. Contrary toexpectation, especially at lower temperatures, such a lamp does notoperate satisfactorily with regard to its igni tion voltage.

Curve 4 applies to a lamp which has an ignition strip 4 mm. wide and thestructure of which is otherwise the same as that of the lamp on whichthe curve 2 was measured. The ignition voltage is generally lower, butthe shape of the curve has remained substantially unchanged. At hightemperatures, this lamp does not operate satisfactorily either.

The curve 5 was measured on a lamp in accordance with the inventionhaving an ignition strip which is 2 mm. wide and is disposed under theluminescent layer, a part of 20 mm. length of this layer situated overthe ignition :strip, measured between the point of intersection of theaxis of the nearest electrode with the luminescent layer and the otherelectrode, being removed. Over a very wide temperature range theignition voltage of this lamp remains within the important region of 205v. to 220 v.

Curve 6 was measured on a lamp in accordance with the invention havingan ignition strip 2 mm. wide, a part of the luminescent layer having alength varying from 20 mm. to 60 mm., measured between a point at adistance of 5 mm. from the point of intersection of the axis of thenearest electrode with the luminescent layer and the other electrode,being removed. Over a wide temperature range the ignition voltage ofthis lamp is lower than 220 v. and even lower than 205 v.

Curve 7 was measured on a lamp in accordance with the invention havingan ignition strip 2 mm. wide, a part of 20 mm. length of the luminescentlayer situated over this strip being removed, while the distance betweenthe end of the resulting window nearest to an electrode and thiselectrode was 100 mm.

Curve 8 was measured on a lamp in accordance with the invention havingan ignition strip 4 mm. wide the structure of which was otherwise equalto that of the lamp on which curve 5 was measured. The ignition as awhole is much lower but for practical purposes such low ignitionvoltages are not always necessary. If the ignition strip is opaque,which is the case, for example, with the strip made of the material ofthe above-mentioned composition, the strip intercepts a greater amountof the radiation emitted by the lamp.

Lamps in accordance with the invention are particularly suitable for usein systems operating without starters.

What is claimed is:

1. A low-pressure mercury vapor discharge lamp comprising an elongateenvelope containing an ionizable medium, a p ir of electrodes positionedat opposite ends of the envelope between which a discharge plasma isformed, a layer of luminescent material supported by the inner surfaceof the envelope, and a conducting strip extending substantially betweenthe electrodes but not connected thereto, said conducting strip having awidth of about 1 to 10 mm. and being disposed between the luminescentlayer and the envelope, said strip having a portion which is inelectrically conducting contact with the discharge plasma, said portionbeing positioned. between at least one electrode and a point spaced fromsaid electrode, the distance between said electrode and said point beingless than one-fifth of the distance between said pair of electrodes.

2. A low-pressure mercury vapor discharge lamp as claimed in claim 1 inwhich the portion of the strip which is in electrically conductingcontact with the discharge plasma is not covered by said luminescentmaterial.

3. A low-pressure mercury vapor discharge lamp as claimed in claim 1 inwhich the portion of the conducting strip which is in electricallyconducting contact with the discharge plasma is covered with saidluminescent material over which a conducting material is applied.

4. A low-pressure mercury vapor discharge lamp as claimed in claim 3 inwhich the conducting material applied to the luminescent layer isidentical with the material of the conducting strip provide-d under theluminescent layer.

5. A low-pressure mercury vapor discharge lamp as claimed in claim 1 inwhich the shortest distance between the electrode and the portion of theconducting strip which is in electrically conducting contact with thedischarge plasma is less than 20 mm.

6. A low-pressure mercury vapor discharge lamp as claimed in claim 1 inwhich the conducting strip does not extend beyond the portion which isin electrically conducting contact with the discharge plasma.

7. A low-pressure mercury vapor discharge lamp as claimed in claim 1 inwhich the length of the portion of the conducting strip which is inelectrically conducting contact with the discharge plasma is between 4mm. and 40 mm.

8. A low-pressure mercury vapor discharge lamp as claimed in claim 1 inwhich the conducting strip is made of opaque material and has a widthless than 4 mm.

References Cited UNITED STATES PATENTS 1/1956 Campbell 313-197 X 7/1964Ray 313221

1. A LOW-PRESSURE MERCURY VAPOR DISCHARGE LAMP COMPRISING AN ELONGATEENVELOPE CONTAINING AN IONIZABLE MEDIUM, A PAIR OF ELECTRODES POSITIONEDAT OPPOSITE ENDS OF THE ENVELOPE BETWEEN WHICH A DISCHARGE PLASMA ISFORMED, A LAYER OF LUMINESCENT MATERIAL SUPPORTED BY THE INNER SURFACEOF THE ENVELOPE, AND A CONDUCTING STRIP EXTENDING SUBSTANTIALLY BETWEENTHE ELECTRODES BUT NOT CONNECTED THERETO, SAID CONDUCTING STRIP HAVING AWIDTH OF ABOUT 1 TO 10 MM. AND BEING DISPOSED BETWEEN THE LUMINESCENTLAYER AND THE ENVELOPE, AND STRIP HAVING A PORTION WHICH IS INELECTRICALLY CONDUCTING CONTACT WITH THE DISCHARGE PLASMA, SAID PORTIONBEING POSITIONED BETWEEN AT LEAST ONE ELECTRODE AND A POINT SPACED FROMSAID ELECTRODE, THE DISTANCE BETWEEN SAID ELECTRODE AND SAID POINT BEINGLESS THAN ONE-FIFTH OF THE DISTANCE BETWEEN SAID PAIR OF ELECTRODES.